An authorized biography of former British Prime Minister Margaret Thatcher, Not For Turning: Margaret Thatcher, The Authorised Biography, Volume One will be released on April 18, the day after her funeral.

The book was announced by Penguin Books UK imprint Allen Lane. Alfred A. Knopf will be the U.S. publisher.

The biography, written by Charles Moore, was commissioned in 1997. Moore had complete access to Thatcher's private papers and early access to her official government papers. He interviewed her multiple times.

"This book also makes her into a three dimensional one for the first time. It gives unparalleled insight into her early life and formation, especially through her extensive correspondence with her sister, which Moore is the first author to draw on. It recreates brilliantly the atmosphere of British politics as she was making her way, and takes her up to what was arguably the zenith of her power, victory in the Falklands."

Proffitt added, "Moore is clearly an admirer of his subject, but he does not shy away from criticising her or identifying weaknesses and mistakes where he feels it is justified. It is, by any standards, an exceptionally impressive book and to be publishing it at this moment is a rare privilege.”

Thatcher did not read the manuscript before her death at age 87 on April 8.